1. INTRODUCTION

It is now thirty years since the first unambiguous
identification of X-ray emission from our nearest giant spiral, M31
(Margon, Lampton &
Cruddace 1974).
Today we know that X-ray emission in galaxies comes
from discrete sources and hot diffuse gas.
Discrete sources (accreting compact objects and SuperNova Remnants) are
a fossil record of the stellar population, and may be used as a probe
of star formation history. Diffuse X-ray emission is the tracer of the
hot gas (T > 106 K)
in galaxies and clusters; in galaxies, it is an indicator of recent
star formation. Disentangling truly diffuse gas from faint,
unresolved sources - such as faint X-ray binaries (XRBs), cataclysmic
variables, coronal emission from main-sequence and T Tauri stars -
is still a major problem, despite the fact that sensitivity and spatial
resolution of the X-ray detectors have improved by three orders
of magnitude since those early observations.

Today, X-ray studies of nearby galaxies can be conducted on two
complementary levels. We can do a statistical study of the spatial and
spectral distribution of the sources: this can help us
distinguish between different physical classes of X-ray emitters.
And we can use the X-ray sources as a tool to probe
the structure and evolution of the host galaxy.

Statistical studies of X-ray sources in the Milky Way are hampered by
the large relative uncertainty in the distance of most sources
(often by a factor of two) and by our very incomplete
view due to the large extinction
in the Galactic plane. Population studies can be conducted more easily
in nearby galaxies, with more favorable viewing angles and the same
relative distance for all the sources. By studying a large sample
of galaxies it is also possible to quantify
how the X-ray properties of a galaxy depend on its structural
type and level of star-forming activity. Moreover, by quantifying the
relation between observed X-ray properties and star formation history
in nearby galaxies, one may predict the luminosity
and color distribution of the faint galaxies
detected in the Chandra Deep Field surveys
(Giacconi et al. 2002),
and therefore probe star formation at high redshift. (See R. Griffiths's
contribution elsewhere in these Proceedings.)

In this conference paper, I have chosen the starburst galaxy M83 to illustrate some of these issues.
Located at a distance of
4 Mpc,
M83 is a grand-design spiral seen at low
inclination. More than 100 sources are resolved in a 51 ks Chandra
observation. In addition to the point sources,
a true-color image (Fig. 1) shows a bright
starburst nucleus and strong, soft diffuse emission along the spiral arms.

Figure 1. A true-color Chandra/ACIS
image of M83 shows
about 130 discrete sources and diffuse emission
in the starburst nucleus and along the arms. The colors are:
red = 0.3-1.0 keV; green = 1.0-2.0 keV; blue = 2.0-8.0 keV. Size of the
image: 12 × 10. North is up, East is left.